The present invention relates to an improvement in the efficacy of herbicidal 2,6-substituted pyridines by combination with a selected second herbicidal compound. The term "2,6-substituted pyridines" is being used herein for such pyridine derivatives which may or may not contain further substituents.
The herbicidal 2,6-disubstituted pyridines to be used according to the present invention are a group of compounds, disclosed in European Patent Applications EP 0 572 093 A, EP 0 692 474 A, EP 0 693 490 and International Application WO 94/22833, which display excellent herbicidal performance, in particular against broad-leaved weeds in cereal crops. However, the 2,6-disubstituted pyridines, when used as the sole active ingredient, do not always achieve effective control of the full spectrum of weed species encountered in commercial agronomic applications, in conjunction with reliable selectivity for the crop species. Such gaps in the spectrum of control can be overcome by co-treatment with another herbicide known to be effective against the relevant weed species. The combined use of certain herbicidal pyridines and in addition other herbicides has been described in International Patent Application WO 94/07368.
Surprisingly, it has now been found, that the combined herbicidal activity of compounds from the above mentioned 2,6-disubstituted pyridines with various partners against many broad-leaved weeds and annual grasses is much greater than expected when applied pre- or post-emergence and that this activity cannot be ascribed to an additive effect, but to a remarkable degree of synergism on many broad-leaved weed species and annual grasses, for example on Setaria viridis, Alopecurus myosuroides, Poa annua, Stellaria media, Lamium purpureum, Galium aparine, Veronica hederaefolia, Papaver rhoeas or Matricaria inodora (i.e. these combinations show a much higher level of activity than predicted from that of the individual compounds) which enables also a greater selectivity for the crop species.
A mixture of herbicides shows synergistic effect if the herbicidal activity of the mixture is larger than the sum of activities of the seperately applied compounds. The expected herbicidal activity for a given mixture of two herbicides can be calculated as follows (See Colby, S. R., "Calculating synergistic and antagonistic response of herbicide combinations", Weeds 15, pp 20-22 (1967): ##EQU1## wherein X is the percentage of growth inhibition upon treatment with a herbicide 1 at a dose of p kg/ha compared with an untreated control (X=0%)
Y is the percentage of growth inhibition treatment with a herbicide 2 at a dose of q kg/ha compared with an untreated control PA0 WE is the herbicidal effect to be expected upon treatment (% of growth inhibition compared with untreated control) with a combination of herbicide 1 and 2 at a dose of p+q g/ha, respectively. PA0 (1) at least one herbicidal component selected from PA0 (2) at least one compound selected from the compounds of general formulae I and II ##STR2## wherein A.sub.1 and A.sub.2 independently represent an aryl group, at least one of A.sub.1 and A.sub.2 being substituted by one or more of the same or different substituents selected from halogen atoms, alkyl groups, alkoxy groups, haloalkyl groups and haloalkoxy groups; PA0 R.sub.1 represents a hydrogen atom, or a cyano group, or an alkyl, alkoxy, alkylthio or haloalkyl group having from 1 to 4 carbon atoms, and PA0 R.sub.2 represents a hydrogen or halogen atom, provided that at least one of R.sub.1 and R.sub.2 represents a hydrogen atom; PA0 n and m independently represent 0 or 1; PA0 A.sub.3 represents an optionally substituted 5 or 6 membered nitrogen containing heteroaromatic group; PA0 A.sub.4 represents an optionally substituted 5 or 6 membered cyclic hydrocarbon, alkyl, alkenyl, alkynyl, aryl or aralkyl group or independently one of the meanings for A.sub.3 ; PA0 R.sub.3 represents a halogen atom or an alkyl, haloalkyl, alkoxy, alkylthio or dialkylamino group; PA0 X represents an oxygen or sulfur atom; PA0 p represents 0, 1 or 2.
If the actual weed control (W) exceeds the expected (calculated) weed control (WE), the mixture displays a synergistic effect.